Integrated Air-Separating And Water-Heating Apparatus Intended For A Boiler

ABSTRACT

An integrated air-separating and water-heating apparatus intended for a boiler is presented.

The present invention relates to an integrated air-separating and water-heating apparatus intended for a boiler.

U.S. Pat. No. 4,461,154 discloses the use of an adiabatic compressor for compressing air and recovering the heat generated at the outlet of the compressor to heat the water which is supplied to a boiler, with the aim of improving the overall efficiency of the air-separating device fed by the compressed air and also of the boiler (for the purpose of reducing the fuel consumption of the boiler).

WO-A-2006/131283 describes a device in which the air from a compressor is heated by flue gases and is then used to heat the water in two separate exchangers.

DE-C-19837251 describes an air-separating device integrated with a gas turbine.

Conventionally, the steam is extracted from a turbine and is then used to preheat the water intended for a boiler.

The present invention enables the heat recovery from an air compressor to be optimized by preheating the boiler feed water.

At the present time, the water supplied to a boiler is sent to a degasser to separate the oxygen dissolved in it, typically in order to reduce the oxygen content to less than 10 ppb by direct steam stripping of the water. In order to be efficient, this degassing must take place at a pressure of less than 20 bar, and preferably less than 10 bar.

When a compressor is used to compress all the air intended for a cryogenic air-separating device, the air must typically be produced at 6 bar abs and therefore at a temperature of 230° C. to 300° C. for an adiabatic compressor.

In theory, therefore, the boiler feed water could be heated to between 220° C. and 295° C. (allowing for the fact that a temperature difference of less than 5° C. would entail significant additional costs).

Two problems need to be resolved.

-   -   In the first place, the water has to be degassed to remove the         dissolved oxygen which mainly originates from the water added to         compensate for the losses (dilution, steam extraction, and         leakage). The pressure must therefore be maintained at a level         of less than 20 bar. The boiling point of the steam at this         pressure is approximately 210° C., and therefore the boiler         water cannot be heated to the optimal temperature in terms of         efficiency (furthermore, a margin of 10° C. must typically be         maintained between the temperature of the water to be degassed         and the temperature of the degasser to enable the latter to         operate correctly).     -   Secondly, the flow of water for the boiler may be too great         compared with the air to allow optimally efficient heat         exchange.

For example, in the case of an air compressor driven by a steam turbine, the relation between the flow of water intended for the boiler (and obtained from the turbine condenser) and the flow of air is 380 kg of water per 1000 Nm³/h of air. The air leaves the compressor at 273° C., the water leaves the condenser at 45° C., and the minimum temperature difference in the exchanger where the water is heated by the air is 10° C.

In this case, the water can only be heated to 224° C., whereas a temperature of at least 250° C. would be desirable.

According to the invention, another heat source is used to complement the heat received from the air compressor, in order to raise the temperature of the water intended for the boiler.

The invention proposes an integrated apparatus, including an air compressor, a steam turbine which drives the air compressor, a first heat exchanger, means for feeding water to the first heat exchanger and from there to a boiler, means for feeding compressed air from the compressor to the first heat exchanger and an air-separating device supplied with air compressed in the compressor, heating means for heating the water downstream from the first exchanger, a second exchanger, means for feeding water from the first exchanger to the heating means, from the heating means to the second exchanger, and from the second exchanger to the boiler, and means for feeding air from the compressor to the second exchanger upstream from the first exchanger, without preheating means between the compressor and the second exchanger, and from the second exchanger to the first exchanger.

In this case, the air from the compressor heats the water without having been preheated by flue gases as in the prior art.

Optionally,

-   -   The heating means are formed by a direct contact heating device.     -   The heating device is supplied with steam from the boiler.     -   The heating device is also used to separate the gases dissolved         in the water.     -   The apparatus includes means for pressurizing the water         downstream from the first exchanger and upstream from the second         exchanger.     -   The apparatus includes an air purification device upstream from         the air-separating device, an auxiliary vaporizer, if required,         for vaporizing a liquid produced by the air-separating device,         means for feeding air from the first exchanger to the air         purification device, means for feeding air purified in the         purification device to the air-separating device, and means for         feeding the water vapor from the boiler to an exchanger for         heating regeneration gas to be sent to the purification device         and/or to the auxiliary vaporizer and/or to an absorption         cooling system of the separating device.     -   The apparatus includes an air purification device, an         air-separating device, an auxiliary vaporizer, if required, for         vaporizing a liquid produced by the air-separating device, means         for feeding air from the first exchanger to the air purification         device, means for feeding air purified in the purification         device to the air-separating device, and means for feeding the         water vapor from the heating means to an exchanger for heating         regeneration gas to be sent to the purification device and/or to         the auxiliary vaporizer and/or to an absorption cooling system         of the separating device.     -   The apparatus includes an air purification device, an         air-separating device, an auxiliary vaporizer, if required, for         vaporizing a liquid produced by the air-separating device, means         for feeding air from the first exchanger to the air purification         device, means for feeding air purified in the purification         device to the air-separating device, and means for feeding water         from the first exchanger to an exchanger for heating         regeneration gas to be sent to the purification device and/or to         the auxiliary vaporizer and/or to an absorption cooling system         of the separating device.     -   The apparatus includes means for feeding water vapor from the         boiler to a steam turbine.     -   The apparatus includes means for condensing the steam from the         turbine and for feeding at least part of the water formed in         this way to the first exchanger, part of the water being fed to         a power plant if required.     -   The apparatus includes means for feeding steam from the turbine         to an exchanger for heating regeneration gas to be sent to a         purification device of the air-separating device.     -   The compressor is an adiabatic compressor.     -   The compressor includes at least one cooling means downstream         from a stage of the compressor.     -   The air is not compressed by any compressor means between the         air compressor (31) and the air-separating device (49).

The invention also proposes a method of heating water intended for a boiler in which water is heated in a first heat exchanger by an exchange of heat with air from a compressor driven by a steam turbine, after which the air cooled in this first exchanger is fed to an air-separating device, characterized in that the water from the first exchanger is reheated and fed to a second exchanger, preferably without having been reheated, where it exchanges heat with air from the compressor, the air from the compressor not being preheated between the compressor and the second exchanger, the air cooled in the second exchanger is fed to the first exchanger, and the water heated in the second exchanger is fed to the boiler.

Preferably, the air compressor produces air at a first pressure and the air is fed to the air-separating device at this first pressure, without compression downstream from the air compressor.

Preferably, all the air from the compressor is fed to the air-separating device.

The invention will now be described more fully with reference to the drawings. FIGS. 1 and 4 show apparatus according to the invention and FIGS. 2 and 3 are Q-T diagrams of an exchanger of the apparatus.

In FIG. 1, water 27 is extracted from a condenser 23 at 45° C. and is pumped to 15 bar by a pump 25. This pumped water is heated by indirect exchange in a first exchanger 29 to a first temperature of at least 100° C., preferably at least 130° C., possibly at least 150° C., or even at least 170° C., for example 175° C. in this case. In this first exchanger, the water recovers heat from the air 35 from an air compressor 31. The compressor can be adiabatic or can have cooling means between the stages. The hot water is fed to a degasser 3 which receives water vapor 5 at 14 bar from a boiler 1. The water is thus heated from its first temperature to 196° C. and dissolved oxygen is removed by stripping. The water 7 at 196° C. is pumped by the pump 9 to 150 bar, creating the flow 11, and is fed to a second exchanger 13 where it exchanges heat with air 33 from the air compressor 31. After passing through the two exchangers, the air 37 is fed to an air-separating device which is illustrated in FIG. 4.

The flow of water at high pressure 15 is fed to the boiler 1. The water vapor 19 from the boiler 1 is fed to a steam turbine 17 which drives the air compressor 31. The steam 21 is then fed to the condenser 23.

The air is not compressed between the outlet of the compressor 31 and the inlet of the air-separating device 49.

FIG. 2 illustrates the heat exchange in the two exchangers 13 and 29. This configuration permits good utilization of the heat from the compressor and efficient degassing at medium pressure.

In order to optimize the exchange diagram (making the lines on the graph as nearly parallel as possible) to obtain the diagram shown in FIG. 3, it is advisable to feed some of the water 27 condensed after the steam turbine 17 of the air-separating device to the preheating system of the power plant, and not to the system for preheating by exchange with hot air.

FIG. 4 is an illustration of a version of FIG. 1, showing the air-separating device in greater detail. The air 37 from the compressor 31 is fed to a purification device 41, and from there to a cryogenic distillation air-separating device 49. In some cases, a liquid product from the separating device 49 is vaporized in an auxiliary vaporizer 51.

The purification device is regenerated by a flow of nitrogen 43 from the air-separating device 49. This nitrogen flow can be preheated by water vapor from the boiler 1 and/or by water vapor 55 from the degasser 3 and/or from the boiler blow-offs. Preferably, the water vapor from the boiler 1 is a fraction of the flow 5 to be sent to the degasser 3.

Additionally or alternatively, a part 57 of the water heated to approximately 150° C. in the first exchanger 29 can be used to heat the regeneration nitrogen 43. This water can be drawn off continuously and stored in a thermally insulated store (not shown) and sent when required to heat the regeneration nitrogen.

Water vapor 53 from the degasser 3 can be used to vaporize a cryogenic liquid of the air-separating device in an auxiliary vaporizer 51.

Part of the water vapor 5 and/or of the water 57 and/or of the water vapor 45 and/or of the water vapor 55 can also be used to heat an absorption cooling unit of the air-separating device 49.

The air is not compressed by any compressor means between the air compressor 31 and the air-separating device 49, and all the air from the air compressor 31 is fed to the air-separating device 49. 

1-10. (canceled)
 11. An integrated apparatus comprising an air compressor, a steam turbine coupled to the air compressor, a first heat exchanger, a means for feeding water to the first heat exchanger and from there to a boiler, a means for feeding compressed air from the compressor to the first heat exchanger, an air-separating device supplied with air compressed in the compressor, a heating means for heating the water downstream from the first exchanger, a second exchanger, a means for feeding water from the first exchanger to the heating means, from the heating means to the second exchanger, and from the second exchanger to the boiler, a means for feeding air from the air compressor directly to the second exchanger upstream from the first exchanger, without passing the air through a preheating means, and a means for feeding the air from the second exchanger to the first exchanger.
 12. The apparatus of claim 11, wherein the heating means are formed by a direct contact heating device.
 13. The apparatus of claim 12, wherein the heating device is supplied with steam from the boiler.
 14. The apparatus of claim 12, wherein the heating device is also used to separate gases dissolved in the water.
 15. The apparatus of claim 11 further comprising a means for pressurizing the water downstream from the first exchanger and upstream from the second exchanger.
 16. The apparatus of claim 11 further comprising; an air purification device upstream from the air-separating device, an auxiliary vaporizer for vaporizing a liquid produced by the air-separating device, a means for feeding air from the first exchanger to the air purification device, a means for feeding air purified in the purification device to the air-separating device, and a means for feeding the water vapor from the boiler to an exchanger for heating regeneration gas to be sent to the purification device and/or to the auxiliary vaporizer and/or to an absorption cooling system of the separating device.
 17. The apparatus of claim 11 further comprising; an air purification device, an air-separating device, an auxiliary vaporizer for vaporizing a liquid produced by the air-separating device, a means for feeding air from the first exchanger to the air purification device, a means for feeding air purified in the purification device to the air-separating device, and a means for feeding the water vapor from the heating means to an exchanger for heating regeneration gas to be sent to the purification device and/or to the auxiliary vaporizer and/or to an absorption cooling system of the separating device.
 18. The apparatus of claim 11 further comprising; a means for feeding water vapor from the boiler to the steam turbine, and a means for condensing the steam from the turbine and for feeding at least part of the water formed in this way to the first exchanger.
 19. The apparatus of claim 11 wherein the air is not compressed by any compressor means between the air compressor and the air-separating device.
 20. A method of heating water intended for a boiler, comprising; heating water in a first heat exchanger by an exchange of heat with air from a compressor driven by a steam turbine, feeding the air cooled in this first exchanger to an air-separating device, wherein the water from the first exchanger is reheated and fed to a second exchanger, where it exchanges heat with air from the compressor, the air from the compressor not being preheated between the compressor and the second exchanger, the air cooled in the second exchanger is fed to the first exchanger, and the water heated in the second exchanger is fed to the boiler. 